Major goals of this project are to gain an understanding of the biology of the diseases that affect rice in California and to develop methods to minimize their damage to the rice crop.  Traditionally the emphasis has been on stem rot and aggregate sheath spot.  However, the presence of the relatively new diseases blast and Bakanae took precedence during 2002 research.  Control methods under study include cultural practices that affect occurrence and severity of diseases and the potential use of fungicides.

Rice Blast

Rice blast disease did not occur in most California rice fields during 2002 and was of little or no consequence in the few fields where it was observed.  New races were not detected.  Researchers believe there is still only one race of the pathogen in the state.

Bakanae

A significant amount of research took place on Bakanae disease last year.  This disease was found in California for the first time in 1999 and has since become widespread in the state.  It was found in all rice-producing counties except Fresno during 2002.  For the most part, the incidence of Bakanae is limited to less than one percent of the total plants in the fields where it is established. In a few cases, however, incidence exceeded 20 percent.

All present evidence indicates that infested seed is the primary source of inoculum and is also the main means of spreading the disease from field to field.  When seed lots have been assayed for presence of the pathogen, the percentage of infested seed has always been higher than the percentage of plants that actually develop Bakanae from that seed lot.

Experiments established in grower fields to determine whether plants could be infected from the pathogen in the soil or in rice residue were inconclusive.  In more controlled tests in basins, with soil known to be free of the pathogen and maintained with known pathogen-free water, there was no development of Bakanae.  In similar experiments where clean seed was planted next to Bakanae-infected plants, there was no spread of the infection.

Field basin experiments comparing the effect of water depth on development of Bakanae revealed that there was no significant difference in Bakanae incidence with depths up to one inch and between four and six inches of constant water flood.  These results suggest that water depth does not affect Bakanae incidence.

Field experiments with seed treated with household bleach were conducted at basins at a research site in Davis.  In all tests, soaking the seed with a five percent bleach solution significantly reduced but did not completely eliminate the disease.

Studies to determine the survival time of Gibberella fujikouri, the microorganism that causes Bakanae disease, in soil and residue thus far indicate that the pathogen survives long enough to carry over between seasons, particularly in situations where rice is grown for consecutive years in the same field.  Fortunately, results indicate that the inoculum from soil and residue plays a minor role in the overall disease cycle of Bakanae.

Studies on the variability and population structure of G. fujikouri in California are being continued.